Lithium-ion batteries continue to dominate the rechargeable battery market. Found in nearly every type of handheld rechargeable phone, music player and many other devices, secondary batteries relying upon lithium metal oxides as the cathode composition eventually experience fade and loss of capacity. Capacity loss increases over the life of the battery necessitating recharging of the battery more frequently.
One well-known mechanism responsible for degradation of the cathode material results from the reaction of electrolyte material with water to form hydrofluoric acid. For example, electrolytes such as LiPF6 react with water to form HF according to the following equation:2LiPF6+6H2O→Li2O+P2O5+12HF.The resulting HF attacks the metal oxides of the cathode. For example, when using a spinel material such as LiMn2O4 (also written as LiMn3+Mn4+O4) as the cathode material, the spinel reacts with HF as represented by the following equation:4H++2LiMn3+Mn4+O4→3λMnO2+Mn2++2Li++2H2O.Since this reaction generates water and in turn additional HF, over time the reaction will completely degrade the cathode material. As the reaction progresses, the manganese ion passes through the separator and becomes part of the solid electrolyte interface (SEI layer) at the anode. The addition of the manganese ions to the SEI layer inhibits the flow of ions contributing to the loss of capacity by the cell.
Other common electrolytes including LiAsF6, and LiBF4, and LiTFSI (lithium bis-trifluoromethanesulfonimide) will also produce HF. Further, alternative cathode materials utilizing first row transition metals such as Co, Mn, Ni, Fe and V (possibly doped with other elements) are equally susceptible to degradation by HF. Accordingly, the ability to shield the cathode material from HF attack without detrimentally reducing battery performance will be commercially advantageous.